Pipeline pigs are used in pipelines to perform a wide variety of cleaning, scraping, inspecting, monitoring, and logging functions. A pipeline pig retriever is used to recover a pipeline pig from a pipeline after it has performed its intended function. U.S. Pat. Nos. 4,709,719 and 6,769,152 provide examples of pipeline pig retrievers. Each pipeline pig retriever has a vertically oriented or horizontally oriented tubular body providing a diversion passage into which the pipeline pig is diverted from the pipeline. The tubular body has a pipeline end and a closure end. The pipeline end is connected to the pipeline and has a valve to selectively connect the diversionary passage of the tubular body to the pipeline or isolate the tubular body from the pipeline. The closure end has a closure that is opened to remove the pipeline pig. There have been a number of accidents in which operators have been injured when retrieving pipeline pigs. In each case, the pipeline pig has become temporarily wedged part way along the tubular body with gas trapped behind it. When the closure was opened and the operator dislodged the pipeline pig, the pipeline pig was propelled rapidly forward by the trapped pressure causing the pipeline pig to strike the operator. What is required is a safe method for detecting and releasing the stored up energy caused by the trapped pressure.